Integrated circuit (IC) "chips" comprising very large numbers of electronic components have become ubiquitous in modem society. Electronic devices and components of all sorts, from central processing units used in all levels of computing, to highly specialized controllers used to control various types of equipment and machinery, are now routinely available as integrated circuit chips. Since the introduction of the first IC chips, there has been a remarkable increase in the number of devices contained on a single chip, as well as a corresponding dramatic reduction in the size of the individual electronic components formed on the chip. Device geometries with line widths of the order of one micron have become common so that individual IC chips now routinely contain in excess of a million electronic components. Even higher device densities are projected.
The increase in device complexity and the decrease in device size has, for many types of IC chips, sharply increased the complexity of forming interconnections between the chips and external devices. These factors, along with a third, related phenomenon, i.e., the increased speed at which many digital devices now function, have increased the heat per unit volume produced by many chips to the point where active cooling methods arc required to avoid thermal damage.
Many devices, such as computers, utilize a large number of separate IC chips. For example a computer may have one or more central processing unit (CPU) chips, various memory chips, controller chips, input/output (I/O) device chips, etc. Traditionally, each chip is mounted in a separate package which is then connected to a printed circuit board, for example, a computer "motherboard", which supplies power to the chip and provides signal routing among the chips on the board and to various I/O devices. However, where an electronic device utilizes a substantial number of chips, packaging each chip separately greatly increases the total area of printed circuit board needed to interconnect all the chips. In addition, as device speed has increased, the distance between individual components has become an increasingly important factor, so that it is important, in many applications, to minimize the signal path between IC chips used in the system.
In order to overcome the aforementioned problems, many device makers have begun using "multichip modules", i.e., packages housing a plurality of individual IC chips. Typical multichip modules incorporate not only means for interconnecting the IC chips with external devices, but also means for interconnecting the IC chips within the module. A general introduction to multichip modules, including a discussion of the history of the development thereof, is described in the text entitled: Multichip Modules: Systems Advantages, Major Constructions and Materials Technologies, R. W. Johnson, et al., eds., IEEE Press (1991). Multichip modules significantly reduce the overall space needed to house the IC chips and, by shortening the distance between chips within the module, facilitate high speed device operation.
The first multichip modules were two-dimensional, i.e., all of the IC chips housed in the package were mounted on a planar substrate. Subsequently, three-dimensional multichip modules were developed, thereby permitting an even further increase in the density of IC chips that could be housed in a single package. However, increasing the number of IC chips housed in a relatively small area further increases the overall heat per unit volume generated by the chip array, while concurrently complicating techniques for actively cooling the chips. Likewise, placing a large number of high density chips in close proximity greatly complicates the task of supplying power to and routing signals to and from the chips. Many of the issues associated with three-dimensional multichip modules are described in a paper by one of the inventors hereof, entitled: "System interconnect issues for sub-nanosecond signal transmission," (L. Moresco) published in Int'l Symposium on Advances in Interconnection and Packaging, Book 2--Microelectronic Interconnects and Packages: System and Process Integration, S. K. Tewksbury, et al., eds., Proceedings of the Int'l Soc. for Optical Eng., SPIE Vol. 1390, (1990). In view of the complicating factors associated with three-dimensional arrays, two-dimensional multichip arrays remain the most common form of multichip modules in use today.
Two major substrate technologies have developed for handling the power supply and signal routing in multichip modules. Initially, co-fired ceramic substrate technology was used but gradually there has been a shift to thin film substrate technology. In either case, a plurality of IC chips are connected to a multilayered substrate which contains all of the signal and power lines needed to supply power and to interconnect the chips to each other and to external devices. In order to make the required number of interconnections, such substrates are multilayered, sometimes containing dozens of individual layers. For example, even early ceramic substrate technology utilized as many as thirty-five separate layers in the multichip substrate. However, problems arise in placing signal lines in close proximity to each other and to power supply lines. The dielectric constant of the substrate material plays an important role in solving (or creating) these problems. As a result, ceramic technology has lost favor due to the high dielectric constant associated with the ceramic materials typically selected for use as a substrate material. Instead low dielectric thin film substrates made of materials such as polyimide have become more common.
In known multichip modules, the individual IC chips are embedded in an encasing material or otherwise sealed. In doing so, it is important that good thermal contact between the chip and the outside be maintained, so that heat does not build up in the chip. A variety of active cooling techniques have been applied to multichip modules, some of them quite elaborate. Frequently, cooling channels are formed in the substrate or elsewhere in the overall structure for forced passage of a cooling fluid. However, in all known devices, cooling of the chips relies on thermal transfer of the heat generated by the chip by conduction through a solid to one or more actively cooled surfaces remote from the chip. This approach is not highly efficient even when a solid with high thermal conductivity is used between the chip and the cooling surface. As a result, heat removal in high power, dense three-dimensional chip modules is still a serious problem.
Another problem with traditional approaches to packaging IC chips in multichip arrays is the method used for delivering power to the chips. As noted above, one aspect of this problem results from routing power lines through the same substrate utilized to carry signals to and from the chip. Equally important is the fact that the thinness of the substrates used in traditional multichip modules results in power feeds to the IC chips that have relatively high impedance. This high impedance results in undesired noise, power loss and excess thermal energy production.
Finally, fabrication yield is a very significant issue in the design and construction of complex multichip modules. In many designs, a failure in any individual component of the module will result in the entire module being useless.
Accordingly, it is an object of the present invention to provide a novel structure for a three-dimensional multichip module whereby the improved cooling of the individual integrated circuit chips is achieved.
Another object of the present invention is to improve the routing of signal and power lines to the integrated circuit chips in a multichip module.
Still another object of the present invention is to provide improved low impedance means for delivering power to the chips of a multichip module.
Yet another object of the present invention is to provide a three-dimensional multichip module design that is highly modular so that the individual components can be pretested prior to final assembly of the module, and such that at least some of said components are replaceable.